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Monocoque is a construction technique that supports structural load by using an object's exterior, as opposed to using an internal frame or truss that is then covered with a non-load-bearing skin or coachwork. The word monocoque comes from the Greek for single (mono) and French for shell (coque). The technique may also be called structural skin, stressed skin, unit body, unibody, unitary construction, or Body Frame Integral (BFI).

Monocoque construction was pioneered in aircraft, with early designs appearing circa 1916, and entering wide use in the 1930s. Automobiles saw moncoque designs as early as 1923, but it wasn't until the second half of the 20th century that widespread adoption began. Today, a welded unit body is the predominant automobile construction technique. Monocoque designs have also been seen in two-wheeled vehicles, water vessels, and architecture.

Aircraft

Early aircraft were constructed using internal frames, typically of wood or steel tubing, which were then covered (or skinned) with madapolam or other fabric to provide the aerodynamic surfaces. This fabric would usually be tautened and stiffened using aircraft dope and was often required to brace the frame in tension but could provide no strength in compression; to resist buckling, these aircraft relied on the rigidity of the internal frame. Some early aircraft designers began to apply sheet metal or plywood to highly stressed parts of the internal frames; this skin did provide strength in shear and compression and could therefore be considered as early examples of monocoque elements but the aircraft still relied primarily on their internal frames.

Design and development

In 1916 LFG introduced their Roland C.II using a fuselage made of moulded plywood, which provided both the external skin and the main load bearing structure. This made the plane very strong for its day, if a little heavy. Similar designs were also produced by Pfalz Flugzeugwerke, who had originally built the Roland under license.

By the late 1920s the price of aluminium, the principal ingredient of the aircraft alloy duralumin, had dropped considerably and duralumin was adopted extensively for internal framing members and later, the skin. It was realised that if the skin were made thick enough it could, theoretically, eliminate the need for any internal framing at all but this would be heavier than an internal frame. Thin sheet metal gauges could easily resist tension and shear loads but buckled under bending and compression. However, if curved, corrugated or rolled into pipe, sheet metal could be made strong against bending and compression loads as well. Stressed skins began to be combined with greatly reduced internal stiffening and came to be what is now known as semi-monocoque.

For example, the Ford Trimotor retained an internal frame of U-shaped aluminium beams but relied on a thin skin of corrugated aluminium sheet to brace this. The corrugations allowed the Trimotor skin to take compression and bending loads, replacing most of the wing ribs and fuselage stringers and could be regarded as a stressed skin structure augmented by an internal frame or semi-monocoque structure. The skin itself had now become a significant structural element in its own right and it was to become even more important when airframes were required to take ever increasing loads.

In the 1930s huge increases in engine power, higher speeds, the need for fuel efficiency and, post World War II, operating altitudes that required aircraft cabins to be pressurised demanded streamlined airframes with stiff, strong, smooth skins; monocoque construction was ideal for this. Torsional (twisting) stiffness was essential to avoid aerolastic deformation under the rising aerodynamic loads. An outstanding early example is the Douglas DC-3. World War II was a major catalyst for aircraft development. At the beginning of the war monocoque construction was in its infancy and many aircraft still used mixed construction or internal frames; by the end of the war, all high-performance planes were monocoque or semi-monocoque.

Aerodynamic considerations of high performance aircraft began to demand the creation of three-dimensionally curved surfaces; a dome is a three-dimensionally curved surface. Happily, any sheet material acquires far more strength when curved in three dimensions as opposed to a simple two dimensional roll such as the Tri-motors corrugated skin. Although expensive to mould, three dimensional shells such as the de Havilland Mosquito moulded plywood fuselage provided immensely strong and light airframes. De Havilland built thousands of wooden monocoque jet aircraft, the Vampires, copying their very successful Mosquito structure.

Limitations

Monocoque construction does not suit all situations. After World War II many lower-performance general aviation aircraft such as the Piper PA-20 Pacer still employed internal frames. Even external wire bracing is still retained for aircraft that have no internal volume such as hang gliders or ultralight, lightly stressed, slow aircraft for which a stressed skin would be too heavy, such as the human powered Gossamer Condor. This is because for very light loads skin stiffness becomes less important than airframe buckling and buckling can be resisted more weight-efficiently by concentrating the compression loads into a few internal struts kept in place by lightweight wire or fabric tension members. Bicycle wheels and tents use tension members in the same way to brace a few compression parts. Monocoque construction can still be more efficient, even in lightly loaded situations, where torsional resistance is the primary need.

An inadequate understanding of early, metal monocoque aircraft design resulted in catastrophic explosive decompressions of pressurised airframes. The two 1954 De Havilland Comet disasters resulted in a major investigation that solved the principal problems of monocoque aircraft construction. It was found that the Comet fuselage failures were caused by a combination of a poor understanding of how stresses are redistributed around openings in a stressed skin, stress raisers such as windows with corners, and an inadequate theory of metal fatigue crack propagation on a pressurised fuselage, that cycles between a pressurised and unpressurised state on every flight. The lessons learnt by the British Royal Aircraft Establishment investigation from the early Comet flaws, were incorporated directly into the design of the Boeing 707 and all subsequent airliners. The oval windows seen on all pressurised jet airliners are one legacy of this. Sound engineering and regular inspections for metal fatigue have made aircraft airframes very reliable. When a crack is found, all parts from that batch are traced and checked carefully or replaced.

Modern technologies

First-stage view of the Falcon I rocket.

The use of composite materials in monocoque skins now allows strength, stiffness and flexibility to be controlled in different directions. Careful design of the direction of the grain of successive layers of materials used in the skin coupled with the use of carbon fibre can produce different mechanical properties in different directions while optimising for weight. Composite materials can be readily built up into complex three-dimensional shapes making them ideal for many aircraft components. They can also be built to be flexible in useful ways, for example, helicopter blades can be made longitudinally rigid but capable of being twisted transversally to adjust the cyclic pitch instead of being mounted on a pivot.

Automobiles

Similar to aircraft, automobile designs originally used body-on-frame construction, where a load-bearing chassis consisting of frame, powertrain, and suspension formed the base vehicle, and supported a non-load-bearing body or coachwork. Over time, this was supplanted by monocoque designs, integrating the body and chassis into a single unit. The external panels may be stressed, in such cases as the rocker panels, windshield frame and roof pillars, or non-stressed, as is often the case with fenders. Today, spot welded unit body is the dominant technique, although some vehicles (particularly trucks and buses) still use body-on-frame.

Early designs

The first automotive application of the monocoque technique was 1923's Lancia Lambda, but it was not a true monocoque because it did not have a stressed roof, it was akin to a boat and has been described as 'punt' type construction. In 1928 German motorcycle manufacturer DKW launched their first car, the P15 wood and fabric bodied monocoque car. The Airflow and Traction Avant steel monocoque cars (with stressed roofs) were both launched in 1934. General Motors then followed with the Opel Olympia in 1935. In 1936, Lincoln introduced the Zephyr, a monocoque design which was as strong as the Airflow yet much lighter.

A halfway house to full monocoque construction was the 'semi-monocoque' used by the 1930s designed Volkswagen Beetle. This used a lightweight separate chassis made from pressed sheet steel panels forming a 'platform chassis', to give the benefits of a traditional chassis, but with lower weight and greater stiffness. This chassis was used for several different models. Volkswagen made use of the bodyshell for structural strength as well as the chassis - hence 'semi-monocoque'.

Nash Motors introduced this type of construction in 1941 with the new 600, generally credited[citation needed] with being the first popular mass-produced unibody construction automobile made in the United States. The all-welded steel with sturdy bridge-like girders that arched front to rear made for improved strength, safety, and durability. Nash engineers claimed that about 500 pounds of excess weight was cut out (compared to body-on-frame automobiles) and the body's lower air drag helped it to achieve better gas mileage. The company's 1942 news release text attached to the X-ray drawing describes how "... all auto bodies will be built ... as this some day..."

Post-war advancements

After World War II, the technique became more widely used. The Alec IssigonisMorris Minor of 1948 featured a monocoque body, as did the Hudson Motor range of the same time. Other automakers incorporated this type of construction, and the terms unit body and unibody became more common in general use. The Ford Consul was the first Ford built in England using a unibody.

In 1960, a major breakthrough in unibody construction was reached in mass-produced Detroit vehicles with over 99% of Chrysler vehicles produced that year being fully unitized; some of the basic designs surviving almost untouched through the mid 1970s (EG: Valiant, Dart, etc.) with tens of millions eventually produced. Convertible versions needed special supports welded underneath to compensate for the missing shape on the top.

American Motors (AMC) continued its engineering heritage from Nash and Hudson, in 1963 combining separate parts into single stampings. The Rambler Classic had "uniside" door surrounds from a single stamping of steel that reduced weight and assembly costs, as well as increasing structural rigidity and improving door fitment.

Hybrid designs

Some American automobiles, such as the 1967-81 Chevrolet Camaro and Pontiac Firebird, 1968-79 Chevrolet Nova and many larger Chrysler Corporation RWD automobiles from 1965 until 1989, used a compromise design with a partial monocoque combined with a rubber-isolated subframe carrying the front end and powertrain. The intention was to provide some of the rigidity and strength of a unibody while easing manufacture. Results were mixed, in large part because the powertrain subframe contained the greatest single portion of the vehicle's overall mass, and thus movement of the subframe relative to the rest of the body could cause distortion and vibration. Subframes or partial subframes are still sometimes employed in otherwise monocoque construction, typically as a way of isolating the vibration and noise of powertrain or suspension components from the rest of the vehicle.

Modern monocoques

In automobiles, it is now common to see true monocoque frames, where the structural members around the window and door frames are built by folding the skin material several times. In these situations the main concerns are spreading the load evenly, having no holes for corrosion to start, and reducing the overall workload. Compared to older techniques, in which a body is bolted to a frame, monocoque cars are less expensive, lighter, more rigid, and can be more protective of occupants in a crash when appropriately designed. The use of higher strength steels in panels at points of high stress has increased strength and rigidity without increasing weight.

In sophisticated monocoque designs, the windshield and rear window glass is bonded in place and often makes an important contribution to the designed structural strength of automobiles.

Some parts of the skin like the grill, the bumpers, the fenders, front wing and rear diffuser are so far away from any load paths that they only hold themselves. The doors and the hood can only transfer a limited amount of load across their gaskets, hinges, and bolts in normal driving situations. The rear door is both far away from any load paths and separated by a gasket. The rear door is a mini-monocoque made of the glass window and the metal frame.[citation needed]

Monocoque designs are favored amongst high-performance cars and racing cars today for their overall structural integrity and the fact that one can design a monocoque out of lightweight materials such as carbon fiber and expect the resulting vehicle to be light, stiff, and stable at high speeds and in tight corners. These types of particularly advanced monocoques can even be molded to create diffusers and ground effects which generate huge amounts of downforce.[citation needed]

Disadvantages

Unfortunately, when a vehicle with a unibody design is involved in a serious accident, it may be more difficult to repair than a vehicle with a full frame. Rust can be more of a problem, since the structural metal is part of the load bearing structure (of metal that is much thinner than a conventional chassis) making it more critical, and must be repaired by cutting-out and welding rather than by simply bolting on new parts (as would be the case for a separate chassis). Structural rust of monocoque cars was a serious problem until the 1990s. Since then, more and more car makers have adopted protection techniques such as galvanizing for structural areas or for the whole body.

Older cars with separate chassis can still pass vehicle inspection tests (such as the British MoT) with quite advanced rust in the sills (rocker panels) and pillars, whereas in more modern cars these parts are structural and would lead to a test failure. In the United States, in the majority of the states which require safety inspections, vehicles will not pass inspection if rust has perforated components such as rocker panels, floor pans, or pillars - regardless of the type of body construction.[citation needed]

Two-wheeled vehicles

LCR Sidecar in race paddock.

Traditional bicycles are not monocoques; they are classic framed structures. However, carbon fibre monocoque framesets are slowly emerging in high-end competitive bicycles, due to their stiffness and light weight. The American company Kestrel USA pioneered [1] the use of carbon fibre monocoques in bike frame manufacture in the 1980s, and since then the technique has become increasingly widely used. Items such as seat-posts and other components are now employing the same technique.

Motor scooters are often made from a pressed steel monocoque frame. This design leads to a light weight and high fuel efficiency, contributing to their popularity.[citation needed]

Water vessels

Frameless glass fibre reinforced or moulded plastic kayaks and canoes and reinforced concrete yacht hulls are monocoques, larger ships tend to have frames but may be hybrids with stressed skins over frames. A submarine has a massive tubular pressure hull at its heart designed primarily to withstand water pressure but because this is so strong it essentially forms a massive stressed skin and can be regarded as a monocoque.

Architecture

Architects occasionally take advantage of the increasing sophistication of monocoque technology in their building projects. Using monocoque technology in buildings allows for interior spaces without columns and load-bearing walls; this creates more spatial and programmatic openness inside. Notable examples are reinforced concrete shells.

Many 1950s and 1960s UK underground protected nuclear bunkers were constructed as reinforced concrete monocoque structures for their inherent strength, robustness and protective factors. Often described as "underground submarines" in that, if they were dug up and placed in water, they would have floated and stayed waterproof.

Structural Insulated Panels, or SIPs, are a type of preinsulated modular wall system. Formerly called "stressed skin panels", they are monocoques in and of themselves. Fastened together properly, they can yield a monocoque housing structure.